High-pressure die casting (HPDC) is a process where metal (this term being used to refer to both metals and alloys) above its liquidus temperature, i.e., fully liquid metal, is injected into a cavity in a mold at high speed and pressure. HPDC is one of the most economical processes for mass production of cast metal items because of the short time needed for the casting cycle, the ability to make multiple items in a single casting, and the fact that the items leaving the mold may be in final form (or nearly so, requiring minimal finishing). However, the speed of HPDC also generates disadvantages. In particular, the turbulence in the flowing metal can give rise to defects in the cast item such as porosity (voids), oxide inclusions (embedded “rusty spots” which affect metal properties), and cold shot (sections which solidify without bonding to adjacent sections), which are not acceptable for applications that require high strength or leak tightness. Additionally, it is difficult to use HPDC to form metal-matrix composites—composites wherein particles are scattered throughout metal (such as the use of hard silicon carbide particles within metal)—since the particles may segregate from the metal matrix owing to the density variations in the metal up to and during the time it is injected into the mold cavity.
Squeeze casting is an improvement of HPDC where the mold is maintained at higher temperature, and the molten metal is injected upwardly against gravity at a slower speed into the mold cavity. The metal flow is laminar, and fills the cavity progressively, thereby allowing higher-quality items to be cast. However, owing to the longer cycle time and substantially shorter die life of squeeze casting, it can rarely be economically used. Also, as with HPDC, it can be difficult or impossible to cast metal-matrix composites owing to particle segregation.
Semi-solid casting is a processing concept where metal is injected into a mold cavity at a temperature between its liquidus and solidus temperatures—in other words, in a semi-liquid/solid state having a “slushy” or butter-like consistency—with the metal having a globular, non-dendritic microstructure (i.e., its microstructure is formed of adjacent crystalline globes or clumps, as opposed to the interbranched snowflake-like crystals found in dendritic microstructures). Semi-solid casting holds great promise since the semi-solid metal has lower energy demands, is more easily handled, and has less porosity because it does not readily flow turbulently. Additionally, the formed semi-solid metal has a near-net shape, requiring minimal post-molding machining, because it experiences less shrinkage than a liquid metal as it cools. However, the drawback of semi-solid casting is the difficulty in generating the globular microstructure, which is needed to provide most of the foregoing advantages (in particular, the ability to non-turbulently flow while in a semi-solid state).
Thixocasting is a semi-solid casting process where metal billets with globular microstructure are formed (usually using electromagnetic stirring), with these billets later being partially re-melted into a semi-solid state before being injected into a mold cavity. Unfortunately, since the cost of the special billets and the re-melting process is high, thixocasting can cost more than squeeze casting, and this issue has limited the acceptance of thixocasting. Thixomolding is a similar process wherein solid alloy pellets are melted, sheared (generally by one or more rotating screws), and transported forward into a shot chamber, from which the alloy (in a semi-solid state) is injected into a steel mold. While thixomolding can be more cost-effective than thixocasting, it has limitations insofar as the metal injection force tends to be lower than that in HPDC (and thus item quality may be lower than in HPDC), and additionally thixomolding cannot be cost-effectively performed with corrosive metals such as aluminum owing to wear on the thixomolding apparatus.
Rheocasting is another type of semi-solid casting process wherein liquid metal is mixed as it cools into a range between its liquidus and solidus (i.e., until it is cooled into a semi-solid state) to produce semi-solid metal with a globular microstructure. The semi-solid metal is then charged directly into a HPDC press to make items. Conceptually, rheocasting could be a cost-competitive process, but it is often difficult to control the process parameters in such a way that the required globular microstructure is produced: the liquid metal must be cooled relatively rapidly without generating large temperature gradients in the metal. Thus, as of 2005, rheocasting has not yet been perfected to such a degree that it has attained widespread use.
It would therefore be useful to have available processes which allow economical and semi-solid casting to be performed with greater flexibility in process parameters, and with the resulting cast items having quality similar to (or better than) those produced using HPDC, with reduced porosity, reduced shrinkage, and refined grain structure. It would further be useful if such processes could accommodate the processing of metal-matrix composites of uniform quality, i.e., wherein particulates are evenly distributed throughout the metal.